In the initial prototype, the bare reed switch (its enclosure being made of glass!) was just taped to the frame next to where the magnet passes; fortunately there was not much rain or mud, since this 'solution' would have fallen apart pretty fast. The switch was soldered directly to the cable harness, with no possibility of disconnection. This is now history.
I am now using a factory-sealed sensor and I crimped an IP67-compliant connector to ease maintenance.
While the initial cardboard-box prototype has been quite a success, clearly for long-term reliability, waterproof enclosures and connections will be required. I have ordered samples of several connectors to evaluate their fitness for this project.
To connect the reed switch sensor, I decided to try out the EDAC 560. Rated IP67, this should provide more than sufficient protection from the environment -- assuming correct crimping. These are only available in a wire-to-wire variant, and they have a disadvantage: it is not really possible to apply strain relief for the individual wires near the connector.
The second sample is the Weipu SP13, a wire-to-wire, soldered variant. Its clones are abundant on sites such as Aliexpress, but I decided to try the original due to its good availability. This is a 4-pin version, which I wanted to use for connecting the front I2C bus (which carries the OLED screen and, in the future, buttons via a port expander). It feels very well-made, however, as expected, it is quite large, so I will not use it at the moment. Instead, I will try to get my hands on a pair of Weipu's miniature SA6 series (these are 6mm aluminium connectors)
Finally, a pair of 14-pin TE AMPSEAL connectors (1-776262-1 + 776273-1). And boy, are they massive. A wire-to-panel combination, these were candidates for the connection between the "ECU" and the cable harness, but I think I will instead go for the more compact SP21 (21mm round plastic connector). I will need about 10 pins for version 2 of the project, so the AMPSEAL would be an unnecessary overkill (the only smaller variant is 8 pins).